1. Field of the Invention
The present invention is directed to an arrangement for producing tomograms of an examination subject using nuclear magnetic resonance imaging techniques, the apparatus having a basic magnetic field as well as gradient fields and a high-frequency antenna whose inductance forms a resonant circuit together with at least one resonance capacitor.
2. Description of the Prior Art
Systems for producing tomograms of an examination subject, preferably a human body, using nuclear magnetic resonance imaging techniques are known. Such NMR (or MRI) systems have a basic field magnet that aligns the nuclear spins in the human body, gradient coils that respectively generate spatially different magnetic fields, and high-frequency coils for exciting the nuclear spins and for receiving the signals emitted by the excited nuclei. When using such a high-frequency excitation and measuring coil, the inductivity of the coil is connected together with a capacitor to form an LC resonant circuit, the capacitor arrangement then being tuned in accord with the desired frequency. A variable parallel capacitor as well as a series capacitor arranged in the feeder for coupling the resonator to a high-frequency generator are generally provided. This high-frequency generator preferably is an oscillator having a following transmission amplifier. Variable disc capacitors, whose capacitance can be controlled by electric motors, are generally employed as variable capacitors.
Dependent on the size of the examination subject, for example a patient, the resonator is loaded, i.e. damped, to different degrees. When matching the resonator to the high-frequency generator whose intrinsic impedance can, for example, amount to 50 ohms, one thus obtains matching frequencies that are dependent on the degree to which the resonator is loaded. The characteristic frequency of the resonator must therefore be detuned such that the resonant frequency from the rated (desired) value caused by the different load of the resonator is compensated. To this end, a tuning of the resonator is undertaken after the introduction of a patient into the nuclear magnetic resonance tomography apparatus by successively adjusting the series capacitor for load matching and the parallel capacitor for frequency correction until the required tuning is achieved.
The no-load quality of the resonator generally amounts to approximately 500 through 1000. Under the load of a patient, this quality normally drops to values below 100. The power of the high-frequency generator must also be designed for this case. All lower load instances require lower high-frequency powers, i.e. when the full amplifier power is forwarded to the antenna in no-load operation or given a weak load, for example as a result of a child as patient, unacceptable voltage or current peaks can then arise. These could in theory be suppressed by appropriate design of the software; for safety reasons, however, such antennas are designed for this maximum load, which is higher than necessary by roughly a factor of three.